In computer animation, a character generally is defined by a topology, often called a skeleton, and an associated geometry, often called a skin or envelope. The topology includes a number of interconnected elements. Each element in the topology is defined in three dimensions by a position and orientation, which may include direction and rotation. Various algorithms control the relationship between the topology and the geometry to produce the look of a character.
Various techniques may be used to manipulate a character to provide the appearance of animation. Generally, such techniques involve associating one or more animation controls with one or more elements of the topology in a process called rigging. One technique is to specify a series of key frames that describe motion of the character over time, using a combination of inverse and forward kinematics and character rigging techniques. Another technique is to use motion capture data representing the position and orientation of selected elements of the topology of a character over time. Other animation techniques include using constraints, scripts or expressions.
Realistic animation of facial expressions, however, is a difficult problem in computer animation. In contrast with the rest of the body of the character, there are only a few parts of the skeleton that can move, i.e., the jaw and the neck. Thus, all other movement of the face that can be animated involves simulating motion of the soft tissue.
Realistic animation of facial expressions is an important problem in computer animation because the face is the primary means through which emotion is expressed. An artist needs to have animation controls that permit subtle changes to the face to be made. Lack of subtlety or realism limits the ability of an animated character to convey emotion and to be perceived as a realistic actor.
Several techniques have evolved to address the problem of face animation, including creating models of muscle systems to mimic the soft tissue behavior, carefully defining poses of a surface mesh by hand and morphing between such poses, and using motion capture to directly control the three-dimensional position of points of a surface mesh. All of these techniques are labor intensive. For animation that is part of a major motion picture release, such as in POLAR EXPRESS, THE INCREDIBLES, and the GOLLUM character in the LORD OF THE RINGS, realistic and complex animation of faces often involves many months of labor by a team of animators.